1. Field of the Invention
The present invention relates to a propeller thrust transmission device for a watercraft propulsion device, which is a device arranged to transmit a thrust force acting on a propeller to a propeller shaft as the propeller in an outboard motor or an inboard/outboard motor rotates.
2. Description of the Related Art
Conventionally, a propeller thrust transmission device, which is a device arranged to transmit a thrust force acting on a propeller to a propeller shaft as the propeller in a watercraft propulsion device rotates, includes a taper portion and a spacer on the propeller shaft. One conventional propeller thrust transmission device of an exhaust device for an outboard motor and the like is disclosed in FIG. 2 of JP-B-Sho 62-15399.
FIG. 4 of the present application is an enlarged cross-sectional view showing a propeller thrust transmission device for a watercraft propulsion device that is practically equivalent to the propeller thrust transmission device shown in FIG. 2 of JP-B-Sho 62-15399. The propeller thrust transmission device has the following structure. An attachment device 14 of a propeller 13 is fitted in and fixed to an outer shaft portion 11a protruding outward from a gear case 12. The propeller thrust transmission device includes a taper portion 11b near the gear case such that a diameter of an end closer to the gear case is larger than a diameter of an end closer to the propeller, and a spacer 16 in which the taper portion 11b is fitted. This attachment device 14 includes a boss 14a united with a blade portion of the propeller and having an exhaust passage for passing combustion gas of an engine therethrough, a tube-shaped rubber damper 14b press-fitted in the boss 14a, a bushing 14c press-fitted inside a rubber damper 14b, and an end plate 14d. The attachment device 14 is fixed by a nut 15. The spacer 16 is shaped to have a taper hole 16a having an inner surface which corresponds in shape to an outer surface of the taper portion 11b (i.e., the inner surface of the taper hole 16a has a complementary shape to the outer surface of the taper portion 11b) and in which the taper portion 11b is fitted, an end surface 16b touching an end wall 14a′ of the boss 14a, and a smaller diameter tube portion 16c (i.e., having a smaller diameter than an outer diameter of the other portions of the spacer 16) fitted in a hole 14a″ in the end wall 14a′. In this propeller thrust transmission device, the taper portion 11b of the propeller shaft 11 receives a thrust force acting on the propeller 13 through the spacer 16 as the propeller 13 rotates in a normal direction.
However, in the propeller thrust transmission device for a watercraft propulsion device shown in FIG. 4, when a watercraft is propelled at a high speed (when the propeller rotates at high speed), a hull or a stern comes to the surface, a draft line lowers, an upper portion of the propeller is exposed to air, and a water resistance force F acts upward on a lower portion of the propeller 13. In such a situation, this water resistance force F is transmitted from the attachment device 14 of the propeller 13 to the smaller diameter tube portion 16c of the spacer 16. A point on the smaller diameter tube portion 16c of the spacer 16 to which a water resistance force is transmitted is spaced by a distance S from the propeller shaft 11 and is distant from a point of application of an average thrust on a thrust force transmission surface between the taper hole 16a of the spacer 16 and the taper portion 11b engaged with the spacer 16. Thus, a moment effect occurs on a lower portion of the taper portion 11b of the propeller shaft 11 rotating at a high speed due to the water resistance force F and the distance L. This situation causes a precession being a load fluctuation in which a surface of the taper hole 16a of the spacer 16 repeatedly separates from the taper portion 11b and then firmly contacts the taper portion 11b of the propeller shaft 11 during the high speed rotation, which results in the spacer 16 abrading the thrust transmission surface (the taper portion 11b). Further, the concentricity between the boss 14a and the bushing 14c is maintained by the rubber damper 14b, and thus when the watercraft is propelled at a high speed (when the propeller rotates at a high speed), the hull or the stern comes to the surface, a draft line lowers, an upper portion of the propeller is exposed to air, and a water resistance force F acts upward on the lower portion of the propeller 13. In such a situation, a lower portion of the rubber damper 14b is compressed, and as a result, the boss 14a moves up toward the bushing 14c. Therefore, the end wall 14a′ of the boss 14a slides on the end surface 16b. The end wall 14a′ of the boss 14a continuously slides on the end surface 16b of the spacer 16 and this sliding movement causes an abrasion on both sliding surfaces due to rotation of the propeller 13 if there is a water resistance force F acting thereupon. Due to continued abrasion between the sliding surfaces, the end wall 14a′ of the boss 14a and the end surface 16b of the spacer 16 separate below the propeller shaft 11 and contact above the propeller shaft 11. Therefore, because the propeller 13 is rotating, the rotation accompanies repeated separating and contacting between the taper hole 16a and the taper part 11b, as described above. This results in a large intensification of the precession mentioned above and an occurrence of abrasion on the thrust transmission surface (the taper portion 11b) by the spacer 16.